KR20080053718A - Ferritic stainless steel having excellent low temperature formability of welded zone - Google Patents

Abstract

A ferritic stainless steel is provided to improve low temperature formability of the welded zone by refining solidified grains of a welded zone and reducing residual amounts of C and N. A ferritic stainless steel having excellent low temperature formability of a welded zone comprises, by mass percent, 0.008% or less of C, 0.008% or less of N, 1.0% or less of Si, 1.0% or less of Mn, 10.0 to 20.0% of Cr, 0.15% or less of Al, 0.0009 to 0.002% of Ca, 0.01 to 0.5% of Ti, and the balance of Fe and other inevitable impurities. The stainless steel comprises Ca-based oxides and Ti-based precipitates. The stainless steel comprises Ca-Ti based oxides. The stainless steel comprises grains with a grain size of 300 mum or less in the welded zone and has 10 Hv or less of a hardness difference between the welded zone and a base metal when the stainless steel is applied to the welding operation.

Description

Ferritic stainless steel having excellent low temperature formability of welded zone}

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram of Sn quench solidification test of the GTA welding part which apply | coated oxide.

Figure 2 is a quench coagulation structure photograph of the GTA welding coated Ca oxide.

Iron and Steel (Vol. 66, 1980, 110p)

2. Japanese Laid-Open Patent Publication 1997-125209

3. JP 2000-160299

4. JP-A 1997-217151

5. Japanese Laid-Open Patent Publication 1997-271900

6. JP-A-1998-324956

7. JP 2001-020046

8. JP 2001-181808 A

9. Japanese Patent Application Laid-Open No. 2001-288543

10. Japanese Laid-Open Patent Publication 2001-294991

11. Japanese Laid-Open Patent Publication 2002-285292

12. Japanese Laid-Open Patent Publication 2002-336990

13. Japanese Laid-Open Patent Publication 2003-221652

The present invention relates to a material having excellent low temperature workability of a weld. More particularly, the present invention relates to a ferritic stainless steel that can improve the low temperature workability of a weld by miniaturizing the solidification grains of the weld and reducing the amount of residual C and N.

Recently, as interest in global environmental issues increases, automobile engine efficiency is required as a means of overcoming strict exhaust gas regulations. Accordingly, the temperature of the combustion gas increases and the operating temperature of the exhaust gas purification system increases. Doing. As a result, materials applied for automobile exhaust systems require even better heat and corrosion resistance.

Austenitic stainless steels have higher temperature strength than ferritic stainless steels, but due to their high thermal expansion, they have a large thermal strain and are produced because they produce thermal fatigue when repeated heating and cooling, and they contain a lot of Cr and Ni. The high unit cost has been pointed out as a problem. On the other hand, ferritic stainless steel is cheaper than austenitic stainless steel, and has been widely applied to automobile exhaust systems, kitchens, and building materials due to its excellent corrosion resistance and thermal fatigue characteristics.

Ferritic stainless steel sheet used as a structural material is often used as welding, it is very important to ensure the quality characteristics of the weld. As an example, the material for the exhaust system is a product obtained by processing a steel sheet or a welding pipe (pipe manufactured by a method such as high frequency welding, GTA welding, laser welding, etc.) into a predetermined shape and then welding again. Since the shape of the exhaust system is quite complicated, during forming, the steel sheet or pipe is subjected to harsh processing. The ferritic stainless steel pipe material has welding cracks in the weld metal or weld heat affected zone when the welding part is subjected to secondary processing such as bending or expansion of the welded part. In many cases, the excellent workability cannot be exhibited. This phenomenon occurs when brittle cracking is prominent in the pipe welded part when forming in winter or at low processing speed.

It is reported that brittle cracks in welds are caused by residual stress, hardening, grain coarsening, and the like. Residual stresses are generated through the deformation after pipe welding and through the sizing roll and correcting the pipe shape. As a method of reducing the residual stress, it is most effective to anneal the entire pipe to remove the deformation amount near the welded portion. Japanese Laid-Open Patent Publication No. 1997-125209 discloses a method of annealing a pipe manufactured by welding at a temperature range of 850 to 1000 ° C and cooling at a cooling rate of 1 ° C / sec or more. According to this method, it is reported that the workability and toughness of the pipe can be improved to the extent of the cold rolled annealing plate. However, when annealing is performed, an increase in the manufacturing cost is inevitable, and in the case of a high alloyed pipe for securing high heat resistance and oxidation resistance, sufficient quality characteristics cannot be secured by annealing.

As a method of controlling the hardening phenomenon of a welded part, it is necessary to reduce the amount of C and N which are impurity elements. Impurities C and N are known to form nitrides or carbides by adding elements such as Ti, Nb and Zr as stabilizing elements together with improvements in steelmaking processes such as VOD (Vacuum Oxidation Decaburizaton) refining techniques. At present, the amount of C + N in the VOD process is about 100 ppm, and the increase in productivity and manufacturing cost due to the addition of the steelmaking process is pointed out as a problem.

To reduce the amount of solid solution C and N by forming nitrides or carbides by adding stabilizing elements such as Ti, Nb, and Zr, it is basically added at least 8 times of C + N. In some cases. However, when a large amount of Ti, Nb, or Zr is added, coarse oxidation inclusions are formed to easily cause surface defects during rolling, and in the case of Zr, nozzle clogging or the like occurs during steelmaking. In addition, in the case of rapid heating quenching such as a welded part, since the time for generating a precipitate is short, rather, the amount of Ti, Nb, Zr, C, N, etc., in solid solution may increase, and processability may not be sufficiently secured.

On the other hand, methods for improving the workability by miniaturizing the solidification structure of the steel and weld metal parts have been recently proposed. The method of controlling the coagulation structure can be largely classified into techniques for improving ferrite nucleation by adding equipment and improving alloys, such as electromagnetic induction stirring of molten steel (Vol. 66, No. 6, 1980, page 38). . In the case of the method by electromagnetic induction stirring, it is known that the equiaxed crystallization rate of about 40 to 60% of the steel is secured by optimizing the stirring position of the molten steel during the solidification. The above technique can improve the machinability of the steel, but it is pointed out that a problem cannot be obtained when remelting the steel such as welding.

The microstructure of the coagulation structure using inclusions is known using TiN (Nos. 1 and 2) and oxides (Nos. 3 and 10). In addition, in the following description, (%) represents wt%.

1. Iron and steel (Vol. 66, 1980, 110p): A method of producing TiN by containing 0.4% Ti and 0.016% N in steel and managing molten steel superheat degree ΔT below 40 ° C.

2. JP2000-160299: A technique for securing an equiaxed crystallization rate of 60% in the slab phase, containing 0.01% or more of TiN inclusions alone.

3. JP1997-217151, JP1997-271900: A technique for miniaturizing the solidification structure of a weld by adding Mg-Al composite oxide by adding 0.001-0.02% Mg and 0.001-0.2% Al, respectively.

4.JP1998-324956: After deoxidizing the amount of oxygen to 0.01% or less in molten steel, adding Mg to 0.0005 ~ 0.01% and starting to solidify within 180 seconds. contained in steel with a distribution of / mm ² .

5. JP2001-020046: A composite inclusion of Mg-Al-based oxide and Ti-based nitride was formed by setting the content ratio of Mg and Al in steel to 0.3 to 0.5.

6. JP2001-181808: Method of improving workability of materials without adding cold rolling by adding 0.0005 ~ 0.01% Mg, minimizing coagulation structure with Mg inclusions and optimizing hot rolling conditions.

7. JP2001-288543: A method for improving the coagulation grains of steel and improving the workability, surface properties and corrosion resistance by adding Mg and Ca content of 0.006% or less.

8. JP2001-294991: Composite inclusions of Mg-based oxides and TiN precipitates contained in steel in a distribution of 0.01 to 5 µm and 3 / mm ² or more.

9. JP2002-285292: Add 0.001 ~ 0.05% of rare earth metal Y to form inclusions such as Al-Y, Mg-Y, Al-Mg-Y and refine solidification crystal grains in steel sheet manufacturing process and steel pipe manufacturing process How to prevent brittle cracks.

10.JP2002-336990: Add 0.01 ~ 0.3% Ti and 0.01 ~ 0.2% Al, and use Ti, Al-based nitride in the weld metal more than 0.3㎛ by using Ar, O ₂ , CO ₂ , He, etc. How to distribute more than 1.5 × 10 ⁴ / mm ² .

11. JP2003-221652: Contains 0.0003 ~ 0.003% Ca and 0.01% or less of O and optionally adds 0.01 ~ 0.3% of Zr to form CaS or CaO oxide in steel material. How to Promote Nucleation Roles.

In the above technique, Nos. 1 and 2 are used to refine TiN in the molten metal to refine the solidification structure of the steel slab, but it is difficult to apply when the temperature control of the molten metal is difficult, such as welding, and a large amount of Ti and TiN impairs the toughness of the steel. Therefore, there is a possibility that the problem of brittle cracking of ferritic stainless steel is further exacerbated.

Known techniques 3 to 10 are methods of promoting the coagulation nucleation by forming oxides in molten metal by adding Mg, Y, etc. alone or in combination, but it is difficult to predict the recovery rate when Mg, Y, etc. having excellent oxidation reactivity are added to molten steel. Therefore, quality deviations by steel materials frequently occur, and there are problems in handling such as explosiveness.

Known technology No. 11 is a method of producing CaS and CaO in molten steel and promoting the production of ferrite (111) plane having excellent workability during hot rolling. However, this causes a problem in that the formation of coarse sulfide CaS lowers the surface quality of the steel and causes corrosion resistance due to the increase in the interfacial area between the inclusions and the matrix.

As such, there is a need for a ferritic stainless steel that can improve weldability within a range that does not reduce the corrosion resistance of the steel, but an alternative has not been proposed.

An object of the present invention is to provide a ferritic stainless steel excellent in low temperature workability of a welded portion.

Ferritic stainless steel of the present invention for achieving the above object is by mass%, C: 0.008% or less, N: 0.008% or less, Si: 1.0% or less, Mn: 1.0% or less, Cr: 10.0-20.0%, Al : 0.15% or less, Ca: 0.0009 to 0.002%, Ti: 0.01 to 0.5%, remaining Fe and inevitable impurities.

Hereinafter, the present invention will be described in detail.

The present invention is characterized in that the solidified crystal grains of the weld portion is refined to improve the low temperature workability of the ferritic stainless steel welded portion, and sufficient carbonitrides are formed to reduce the amount of residual C and N.

Hereinafter, the reason for component limitation of the ferritic stainless steel of this invention is demonstrated.

C, N: C, N is an invasive element that deteriorates the workability of the base metal and the welded part. Therefore, it is preferable to minimize the amount of C, N: 0.008% or less, N: 0.008 It should be less than%.

Si, Mn, Al, P, S: These elements are invariably present in steel, but when present in a large amount, they lower workability and lower corrosion resistance, which is characteristic of stainless steel, Si: 1.0% or less, Mn: 1.0% or less , Al: 0.15% or less, P: 0.040% or less, S: 0.010% or less is appropriate. Particularly, in the case of S, CaS may be formed to cause a large problem in corrosion resistance. Therefore, it should be controlled within the above range.

Cr: Cr is less than 10%, and the Cr content is 10% or more because the corrosion resistance, which is the basic characteristic of stainless steel, is insufficient. If the Cr content is high, the toughness of the weld section may deteriorate, so the Cr content is 20% or less.

Ca: Ca is an essential element for improving the weldability which becomes a subject in this invention. In order to improve weldability, addition of 0.0009% or more is required. However, if it is added in excess of 0.002%, the upper limit is set to 0.002% because the size of the oxidation inclusion increases and adversely affects the corrosion resistance.

Ti: It is added as an element which improves workability, and an effect appears by adding 0.01% or more. However, when added in excess of 0.5%, there is a problem that workability deteriorates due to an increase in the amount of solid solution Ti.

The alloying element may be included according to the properties required for the steel formed from the above component system. For example, in order to improve corrosion resistance, at least one of Mo, Ni, and Cu may be further added in a composition range of 0.1-2.0%. When the content of at least one of Mo, Ni and Cu is more than 0.1%, the effect of improving corrosion resistance can be obtained. If the content exceeds 2.0%, the workability is deteriorated and the manufacturing price is increased. In addition, Nb may be included up to 0.5%. If the content of Nb exceeds 0.5%, there is a problem that the workability is deteriorated by an increase in the amount of solid solution Nb. When Nb is added to form NbN, NbC, or the like to improve workability, it is preferably included as 0.01-0.5%.

Ca-based or Ca-Ti-based oxide: Ca-based or Ca-Ti-based oxides are present in the steel that satisfies the composition range according to the present invention. Since the oxide has a high melting point and is present in the newly solidified tissue even after being welded again after being formed in the molten metal phase, the oxide acts as a solidification nucleus, and therefore, the present invention is particularly effective in improving low temperature workability of the weld. Ca has a volatility, so there is a problem that the amount remaining in the electric furnace process, AOD, VOD process is reduced, it is preferable to add immediately before the continuous casting process.

Ti-based or Nb-based precipitates: Ti or Nb reacts with carbon in the molten metal to form precipitates in the form of TiC or NbC to improve workability by reducing residual C and N amounts (free C, N). The main component of the precipitate may be carbides or some nitrides. Ti is added before the refining step and is added before Ca. However, in the case of TiN, the melting point is lower than that of Ca- or Ca-Ti-based oxides, so that TiN surrounds some of the composite oxides after Ca- or Ca-Ti-based oxides are formed. Form to form a composite placement. In addition, in the case of TiN and TiC, the melting point is low, and when heated and quenched like a welded part, there is a problem of returning to the remaining C and N again. However, in the present invention, a large amount of TiN and TiC had to be added. Solved. In the case of Nb, nitride is formed in the same manner as Ti to form a composite inclusion with Ca-based or Ca-Ti-based oxide, which can be selectively or combined with Ti.

Hereinafter, the present invention will be described in detail with reference to the following examples, which are only preferred embodiments of the present invention, and the scope of the present invention is not limited by the description of these embodiments.

(Example)

In order to investigate the effect of the addition of various oxides on the workability of the ferritic stainless steel welded portion, as shown in FIG. 1, various oxide powders having a size of 1 μm were coated on a steel sheet, subjected to GTA welding, and then quenched and solidified in Sn molten metal. . FIG. 2 shows that ferrite solidification starts preferentially around Ca-based oxides when Ca-based oxides are added. That is, when high temperature oxide is present in steel, ferrite solidification starts around the oxide during welding, so it is determined that the solidification crystal grains of the weld portion are equiaxed and refined.

Table 1 shows the results of measuring DBTT (ductile-brittle transition temperature) and hardness of welded metal parts by grain size, isotropic crystallinity, and Charpy impact test after coating various oxides on steel and performing GTA welding. to be. It can be seen that the addition of Ca and Zr oxides resulted in finer grains and improved DBTT characteristics of the welded portions than the addition of additives and Mg oxides. The addition of Zr-based oxides greatly improves the quality characteristics of the welds, but shows a problem that the tungsten electrodes are severely damaged during welding. From this result, it can be seen that when welding ferritic stainless steel, Ca-type oxide is included in the steel material, the low temperature workability of the welded part can be improved.

division Grain size (㎛) Isometric rate DBTT (℃) Hardness (Hv) Additive-free 530 1.7 -18 158 Ca-based oxide 138 1.4 -38 168 Mg oxide 183 2.38 -28 170 Zr oxide 131 1.3 -34 167

Six types of ferritic stainless steels shown in Table 2 were manufactured using field production facilities. First, a plate of 1.5 mm in thickness of the final product was produced through solvents in an 80 ton electric furnace and hot rolling, annealing, pickling, cold rolling, annealing, and pickling. In the refining process, the amounts of impurities C and N were controlled, and Ca was added to the ladle melt while varying the amount of Ca-Si alloy. Ca-Si alloys were charged in the form of Lumps or wires.

No C N Nb Si Mn P S Cr Ni Ti Ca One 0.007 0.008 0.013 0.45 0.30 0.01 0.002 11.3 0.09 0.23 - 2 0.006 0.006 0.013 0.45 0.31 0.01 0.002 11.3 0.09 0.23 0.0004 3 0.006 0.006 0.013 0.42 0.31 0.01 0.002 11.3 0.09 0.24 0.0009 4 0.007 0.007 0.013 0.45 0.30 0.01 0.002 11.3 0.09 0.23 0.0014 5 0.008 0.009 0.012 0.43 0.31 0.01 0.003 11.2 0.09 0.22 0.0013 6 0.006 0.007 0.012 0.43 0.30 0.01 0.003 11.4 0.08 0.22 0.0022

GTA welding was performed using a DC type welding machine (maximum welding current 350A), and a bead on plate. Welding conditions were welding current 110A, welding speed 0.32m / min, tungsten electrode diameter: 2.5mm, electrode tip angle: 100 ^o , Arc length 1.5mm, protective gas Ar (15l / min).

Tube welding is performed with welding condition 180A, pipe speed 1.0m / min, tungsten electrode diameter: 2.5mm, electrode tip angle: 100 ^o , Arc length 1.5mm, protective gas Ar (15l / min), pipe outer diameter is 50.8mm, The thickness was produced to 1.5mm. In addition, the back shielding treatment was performed using Ar gas to prevent the mixing of external air in the lower part of the welding bead.

Grain size of the weld was measured using an optical microscope. The weld cross section was polished using sandpaper and an abrasive, and observed after electrolytic etching with a Nital solution. The hardness distribution of the welded part was measured using a MicroVickers hardness tester with a load of 200 g and a holding time of 10 s at 0.2 mm intervals.

The DBTT characteristics of the welded portion were investigated by applying the Charpy impact test on a 1/4 Sub-size (1.5 mm ^t × 10 mm ^w × 55 mm ^l ) test specimen in the test temperature range of -60 ° C to 50 ° C.

The workability of the pipe material was evaluated as the presence of cracks in the welded part when the pipe was expanded to a limit crack rate of 25% at -20 ° C.

Table 3 is a result of evaluating the quality characteristics of the weld plate and pipe material for the six ferritic stainless steel. No. 1 and No. 2 are cases in which Ca is added or only a small amount is added, No. 3 and No. 4 are examples of the present invention, No. 5 is the case where N amount is exceeded, and No. 6 is Ca If too much addition.

When the amount of Ca is more than 0.0009% and the amount of C + N is not more than 0.016%, as in Nos. 3 and 4, the grain size of the weld and the hardness difference between the weld and the base material are reduced compared to those of Nos. 1 and No2. It can be seen that the DBTT characteristic and impact energy deviation of the welded part are also improved. In the case of a TIG pipe material containing 0.0014% of Ca, cracking did not occur even after bending at -20 ° C. However, in the case of steel material No. 5 containing an excessive amount of C + N exceeding 0.016% even when an excessive amount of Ca additive 6 (No. 6) and Ca are added, it is understood that the weld hardens and the workability is lowered.

No Sheet welded part quality characteristics Pipe welding part quality characteristics Grain size of weld (mm) Hardness Difference (Welding-Material, Hv) DBTT (℃) Impact energy deviation (J, -20 ℃) Crack incidence of welded part (%, -20 ℃) One 278 21 -18 6.06 10 2 530 14 -30 5.1 3 3 270 7 -47 4.8 0.5 4 216 7 -57 2.0 0 5 220 10 -42 4.5 1.0 6 230 15 -35 5.7 2.5

As described above, the ferritic stainless steel of the present invention is useful to provide ferritic stainless steel excellent in low temperature workability of the weld by adding Ca and Ti to refine the solidification grain of the weld and reducing the amount of residual C and N. It works.

Claims (4)

By mass%, C: 0.008% or less, N: 0.008% or less, Si: 1.0% or less, Mn: 1.0% or less, Cr: 10.0 to 20.0%, Al: 0.15% or less, Ca: 0.0009 to 0.002%, Ti: Ferritic stainless steel with excellent low temperature processability of welded parts, which is composed of 0.01 to 0.5%, remaining Fe and inevitable impurities. The ferritic stainless steel of claim 1, wherein the stainless steel comprises a Ca-based oxide and a Ti-based precipitate. 3. The ferritic stainless steel having excellent low temperature workability of a welded portion according to claim 2, wherein the stainless steel comprises Ca-Ti oxide. The ferritic stainless steel having excellent low-temperature workability of the welded portion according to claim 1, wherein when the welding is applied, the grain size of the welded portion is 300 µm or less, and the hardness difference between the welded portion and the base material is 10 Hv or less.

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